Choose the appropriate word pair to complete the sentence: I didn't k…

The correct answer is Option (2) → had known, have visited

Third conditional (unreal past condition):

• It imagines the opposite of a past fact and its result.
• Pattern:
  • if-clause: If + had + past participle
  • main clause: would + have + past participle
• This sentence: "I didn't know Rahul was in the hospital" is about the past, and the speaker did not visit him. So: "If I had known, I would have visited him."
• Complete sentence: I didn't know Rahul was in the hospital. If I had known, I would have visited him.

What happened	Third conditional
I didn't know, so I didn't visit him.	If I had known, I would have visited him.

Why the Other Options are Incorrect?

Option 1: knew, go — Incorrect. "If I knew, I would go" is the second conditional, for an unreal situation in the present or future. The hospital stay is in the past.

Option 3: have known, be going — Incorrect.

• The present perfect "have known" is not used in an if-clause about an unreal past.
• "Would be going" refers to the present or future.

Option 4: know, had gone — Incorrect.

• The tenses are reversed: "would had gone" is not grammatical.
• "Know" is present tense.

Contrast: "If I knew, I would go" (second conditional) is for an unreal present or future. "If I had known, I would have visited" (third conditional) is for an unreal past.